Updated

The number of people killed by police in Brazil's biggest state has plunged 40 percent, and experts are crediting what seems an unlikely reason — a new rule that forbids officers from transporting or otherwise helping shooting victims. They say it makes the summary execution of suspects more difficult as the government moves to end long-standing impunity in such slayings.

On its face, the no assistance rule implemented amid much criticism a year ago would seem contradictory to saving lives. Police were told they could no longer offer first aid to shooting victims, including people they shot themselves, nor could they take them to a hospital.

Yet police officials and watchdog groups alike said this week the rule has helped save lives in two ways: Shooting victims receive better medical care from ambulance crews than they would get from police, and it's now harder for officers to carry away a shooting victim in their car only to execute them in another location.

"Before (the rule) was enacted, those wounded in shootouts were tossed into police vans that would take one to two hours to reach the hospital," said Guaracy Mingardi, a Sao Paulo-based crime and public safety expert. "In several cases, the suspect was executed inside the van taking him to the hospital."

The rule is part of a big change in attitude by officials to crack down on officers who execute suspects.

"Until the end of 2012, law enforcement authorities did not care if police were killing or not killing," Mingardi said. "They then made it clear that police officers who killed in confrontations or executed suspects while being taken to a hospital would no longer enjoy the impunity" they once had, he said.

According to the Sao Paulo state's Public Safety Department, police killed 335 people in 2013, compared to 546 during the previous year.

When the no-assistance measure was introduced in January 2013, Col. Marcos Chaves, the Sao Paulo State Police commander, said it was aimed at making officers' actions more transparent.

"Officers are always seen with suspicion whenever there is a shootout. No one knows if it actually occurred and if the scene of the crime was altered. The new measure will end these suspicions," Chaves said then.

Police in Sao Paulo, Rio de Janeiro and other Brazilian cities in the past have been accused of extrajudicial "resistance" killings, or summary executions of suspects. The U.S.-based watchdog group Human Rights Watch estimated that police killed about 11,000 people in 2003-2009 in Rio and Sao Paulo alone. A 2008 United Nations report found that Brazilian police were responsible for a significant portion of the country's 48,000 slayings the year before.

In its World Report 2014, Human Rights Watch said the measure ordering police to not aid shooting victims helped reduce police killings. But the group warned that much more needs to be done, noting that "significant obstacles to accountability for unlawful killings in Sao Paulo persist."

Those obstacles include "the failure of police to preserve crucial evidence, and the lack of sufficient staff and resources provided to prosecutors responsible for investigating these cases."

Marcos Fuchs, a director of the Brazilian human rights group Conectas, said spending to improve investigative and forensic methods and improve police wages "have inhibited people from committing crimes and this results in less confrontations with police."

Earlier this month, the Sao Paulo state government said it will send the state legislature a proposed law authorizing quarterly bonuses of more than $800 to police officers who reduce crime rates without the use of lethal force. If lawful lethal force is used, the bonus drops to $250.

In 1990s in some Brazilian cities, including Rio de Janeiro, police officers were promoted and paid cash bonuses for engaging alleged criminals in shootouts that resulted in death.